Touch and Proprioception
Anyone who has examined cleaned and dried shark jaws may have noted that the teeth are set quite firmly. Yet in living sharks, some of the teeth are remarkably mobile. Simple manipulation of the anterior teeth of a fresh, whole shark specimen reveals that each tooth tip can be rocked back-and-forth through an angle of ten degrees or more. The tooth which one rocks also causes nearby teeth to move, flexing forward and back as the tough tooth bed is pulled and relaxed. Closer examination reveals that each tooth is supplied with blood vessels and nerves, reminding us that — like our own, often neglected, chompers — shark teeth are living, sensitive organs.
In a jargon-dense but fascinating 1995 paper, Canadian marine biologist James Powlik measured changes in White Shark tooth position during biting. Powlik examined the jaws of fresh Great White heads as well as analyzed high-speed video footage of this species feeding. He found that, as a White Shark's jaws are opened to an angle of 20 to 25 degrees, the upper and lower teeth rotate outward by almost 9 degrees. Further, Powlik found that as a White Shark's jaws close through an angle of 35 degrees or more, the teeth rotate inward by almost 16 degrees. Powlik deduced that this tooth rotation is due to a combination of the jaw cartilages being flexed by the biting muscles and the tooth bed sliding over the jaw. Thus, as a Great White opens its jaws to bite, its teeth rotate outward, and as the shark closes its jaws, its teeth rotate inward — in an action remarkably like nimble human fingers that alternately extend and flex as the hands (representing the jaws) are repeatedly drawn apart and together. Since each tooth is equipped with pressure- and position-sensitive nerves, a White Shark's functional dentition can provide a great deal of tactile information. This acute touch sensitivity, combined with jaws that are highly mobile and can be finely controlled, can provide the Great White with an astonishing dexterity for manipulating and examining objects in its environment.
Thus, lacking hands with which to explore novel objects, a White Shark uses its dexterous and touch-sensitive jaws to 'feel' things. Which brings us back to Ames and Morejohn's mangled Sea Otters. Recall that the White Shark is warm-bodied, having an elevated need for high-quality food. In a 1994 semi-technical article, shark behaviorist A. Peter Klimley hypothesized that White Sharks may reject low-quality food items that lack extensive reserves of blubber. Sea Otters — which rely on their thick, luxuriant fur (over 1 million hairs per square inch or 155 thousand per square centimetre) to keep them warm in the cold waters they inhabit — lack a thick, insulating layer of blubber under the skin. Once bitten, Klimley suggests, Sea Otters may be rejected as low quality food rather than because they smelled or tasted 'bad' to the shark. Paleontologist Douglas Long has observed White Sharks feeding selectively on the blubber of whale carcasses and not their muscle. Klimley has been unable to trick White Sharks into swallowing telemetering (sonic transmitter) devices hidden in relatively lean sheep carcasses, but has had success using blubbery seal carcasses. Klimley has speculated that the Great White may use its touch-sensitive jaws as a tensiometer (a device used to measure surface stress of objects), accepting or rejecting potential food items depending upon how fat-rich they may be.
In addition to having senses monitoring the external marine environment, sharks — like other animals, including humans — have sensors for feeling what is going on inside their bodies. Sensors which provide feedback about such internal things as relative limb or fin position, muscular tension, stomach distention, body temperature, oxygen demand, injuries, and so on are termed proprioceptors. By and large, we tend to take proprioception for granted, thinking it completely unremarkable that, for example, we know at all times where our hands and feet are — even if we cannot see them. But consider the poor octopus. Despite having a remarkable molluscan nervous system, without joints to serve as reference points, an octopus cannot tell where its arms are unless it is looking directly at them. For the most part, proprioception is a deeply individual experience and one which cannot easily be studied or described. Pain, for example, is so intensely personal that it isolates us from everyone around us — even those individuals who care about us most. The problems encountered in understanding the experience of proprioception in others multiply immeasurably when we contemplate it in other species.
We can never know what it 'feels like' to be a Great White or other shark. Often, the best we can do is understand something about how its various sensory systems work, to which stimuli they respond and at what thresholds. Beyond these basics, we can try to correlate what we know about the White Shark's sensory biology with its overt behavior. But we must never allow ourselves to forget that this animal inhabits a medium that is alien to us and lives by voices we can never hear. It is perhaps this unknowable quality that makes the Great White such an irresistible enigma. The profound desire to see the invisible, hear the most perfect of silences, touch the untouchable, and know the unknowable is a fundamental part of the experience of being human. As a species, it seems, few things are as seductive to us as the spectacularly inaccessible. In an ironic yet powerful way, the inscrutable Great White Shark helps us to understand our own uniqueness.